2003-07-24 04:00:00 PDT Washington -- The Sept. 11 attacks were preventable but went undetected because of communications lapses between the FBI and CIA, which failed to share intelligence related to two of the hijackers, a congressional report to be released today has concluded.

The report, by a joint committee of the House and Senate intelligence panels, found that for nearly two years before the attacks, the CIA knew about the terror connections between the two men, Khalid al-Midhar and Nawaf Alhazmi,

who in 2000 moved to San Diego, frequenting Muslim circles that had been infiltrated by FBI.

According to some people who have seen it, the report's central conclusion is that if the CIA had shared its information, and if the FBI had used its informants more aggressively, the presence of al-Midhar and Alhamzi in San Diego offered "the best chance to unravel the Sept. 11 plot."

The report by the joint committee was described as a scathing critique of the FBI and CIA in the months before the attacks, concluding that the country's main intelligence agencies failed to counter the al Qaeda threat even though they had known for years that Osama bin Laden was determined to attack the United States.

The report concludes that neither the FBI nor CIA acted forcefully enough to collect intelligence from informants in the United States and abroad.

Both agencies say they have worked to overcome their communication failings by creating a joint threat assessment unit and by exchanging far more information than in the past.

After nine public hearings and 13 closed sessions conducted last year, the joint committee report is the most comprehensive account yet of lapses that allowed 19 Arab men to hijack four commercial aircraft without being first detected by intelligence or law enforcement authorities. Two of the jets destroyed the World Trade Center in New York City, one crashed into the Pentagon, and the fourth crashed in a field in Pennsylvania after passengers apparently wrested control from the hijackers.

While concluding that the attacks could have been stopped, the report does not blame either agency for overlooking specific information that would have thwarted the attacks.

Providing fresh insights into some key events in the months before the attacks, the report repeats some findings previously released by the joint committee, such as the criticism of the CIA and FBI for their handling of issues related to al-Midhar and Alhamzi. The two Saudi Arabian men lived in the United States undetected until after they boarded American Airlines Flight 77, which crashed into the Pentagon.

In the case of al-Midhar and Alhamzi, CIA officials failed to alert the FBI or place the names of the two men on any watch list until August 2001, a few weeks before the attacks. None of the FBI's informants, some of whom had contact with the two men, were alerted that they might have been al Qaeda members.

The report found that the National Security Agency, which eavesdrops on worldwide communications, might have had the earliest knowledge of the extremist leanings of al-Midhar and Alhamzi from intercepts in early 1999 but had not disclosed that information to other intelligence agencies.

The report is the end result of a behind-the-scenes battle between the committee and the Bush administration over classified information. One lengthy section of the report, relating to the cooperation of foreign governments such as Saudi Arabia, has been deleted from the report, at the insistence of the administration.

Mueller said he had made the prevention of terror attacks the FBI's top priority and had doubled the number of agents and analysts assigned to counterterrorism from 1,500 to 3,000. "No counterterrorism lead goes unaddressed," Mueller said.